1. Field of the Invention
This invention relates to a novel acrylic copolymer, and a reflection-preventing film-forming composition containing the same which composition is useful in fine processing in lithographic processes making use of radiations of various types and is especially suited for the fabrication of integrated circuit devices.
2. Description of the Prior Art
In processes for fabricating integrated circuit devices, the processing size in lithography is being made finer in order to attain a higher integration. In such lithography, a resist composition is coated on a substrate, and a mask pattern is transferred by means of a demagnification projection exposure system (a stepper), followed by development with a suitable developing solution to form the desired pattern. However, substrates of aluminum, an aluminum-silicon alloy, an aluminum-silicon-copper, polysilicon and tungsten silicide used in this process, having a high reflectance, necessarily reflect from their surfaces the radiations made incident thereon. This may cause halation in the resist pattern to cause a problem that any fine resist pattern can not accurately be reproduced.
In order to solve such a problem, it is proposed that a reflection-preventing film having properties of absorbing radiations reflected from the substrate is formed beneath a resist film to be formed on the substrate.
Such a reflection-preventing film is first of all known to include inorganic films such as a titanium film, a titanium dioxide film, a titanium nitride film, a chromium oxide film, a carbon film or an .alpha.-silicon film which are formed by a process such as vacuum deposition, CVD (chemical vapor deposition) or sputtering. These inorganic reflection-preventing films, however, are electrically conductive and hence have disadvantages that they can not be used in the manufacture of integrated circuits and the formation of reflection-preventing films requires a special system such as a vacuum deposition system, a CVD system or a sputtering system. In order to solve the disadvantage inherent in the inorganic reflection-preventing films, an organic reflection-preventing film is proposed which is formed of, e.g., a polyamide acid (co)polymer or a polysulfone (co)polymer and a dye (see Japanese Pre-examination Patent Publication (Kokai) No. 59-93448) (the term (co)polymer is herein meant to embrace a polymer and a copolymer). This reflection-preventing film is not electrically conductive and also is soluble in suitable solvents, and hence can be formed by coating on a substrate in the state of a solution, like a resist, without requiring any special system.
However, the reflection-preventing film formed of a polyamide acid (co)polymer or a polysulfone (co)polymer and the resist may mix with each other (this is called intermixing), though slightly, to cause deterioration of resist pattern shape, e.g., poor clearing and trailing, thus there remains a problem. Also, with an increasing demand for making integrated circuits finer, resists are sought to be made more thin-film, but the above reflection-preventing film has a disadvantage that it is etched at so low a rate (hereinafter "etching rated") that long-term irradiation with plasma gas is required in the step of removing the underlying reflection-preventing film by dry etching, so that the resist layer coated in a small thickness may also be corroded simultaneously to cause a serious film thickness loss of the resist layer consequently.